Waste papers have for many years been an important source of fiber for use, or rather for reuse, in the manufacture of new paper products, and waste paper materials of a variety of types and grades are recognized commodities for purchase by paper and paper board mills. There has in recent years been increasing emphasis on such recycling of used paper materials, and this emphasis has now extended to the recovery of reusable paper fiber from municipal solid waste, by means of the system and method of disclosed in my assignee's recent U.S. Pat. No. 3,736,223.
The recycling of commercially available waste paper has always presented some problems of cleaning and screening because the economics of collecting, sorting and baling of used paper products commonly result in the inclusion of substantial quantitites of inorganic contaminants as well as plastic, rags and the like. The high specific gravity contaminants of waste paper mixtures are not difficult to separate from the paper, by conventional junk removers and centrifugal cleaners. The more serious problems derive from the plastic and other trash materials which have specific gravities so close to that of cellulose fiber that gravity separation is not effective, and which now commonly average from 3-5% to as high as 15% by weight of commercial waste paper.
More specifically, pulpers which are commonly used for initially breaking down waste paper are quite efficient for slushing but less so for complete defibering, and much longer pulping time is required if the pulper is relied upon to reduce the paper to particle sizes capable of passage through relatively small holes, e.g. 3/8 inch in diameter or less. Under such conditions, however, the plastic constitutents of the mix will not be correspondingly reduced in particle size, and they will tend to accumulate in the pulper, and to interfere with proper extraction of the defibered paper. This in turn will result in such loss in the effectiveness of the pulper that frequent stops for cleaning out the pulper become necessary.
When attempts were made in the past to avoid this problem by the use of larger extraction holes from the pulper, other difficulties resulted. For example, since a portion of the paper charge is not completely defibered, many paper pieces will be comparable in size to the fragments of plastic and cannot readily be separated by screening. This problem can be overcome by subjecting the mixed fibers and fragments to treatment in apparatus of the type known as deflakers which will defiber the paper without comparable reduction of the particle sizes of the plastic, but such treatment is inefficient because the defibering apparatus must handle both the plastic, the paper pieces, and the already defibered paper, with resulting wasteful consumption of power.
The most effective system and method previously developed for dealing with this problem are disclosed in my assignee's U.S. Pat. No. 3,873,410 (Chupka). According to that patent, the different stations in the system are so arranged that the pulper is utilized only for slushing rather than for complete defibering of the paper, the slushed and centrifugally cleaned slurry is then screened at high consistency and with a high reject rate to separate such paper as has been essentially defibered from the plastic and remaining paper, and the material rejected by the screen is then directed to a deflaker from which the accepted material is recycled in the system.